In the Standard Model (SM), decays of the charmonium resonances \(J/\psi \)  and \(\psi (3686)\)  (collectively referred to as \(\psi \)  throughout the text) induced by Flavor Changing Neutral Current (FCNC) are forbidden at the tree level due to the Glashow-Iliopoulos-Maiani (GIM) mechanism [1]}, but can occur via a \(c \rightarrow u\)  transition at the loop level, e.g., shown in Fig. REF  for the decay of \(\psi ~\rightarrow D^{0}e^+e^-\) . Such decays can also occur via long-distance effects on the hadron level, which are, according to Ref. [2]}, expected to have the same order of magnitude as the FCNC process.
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